Conventionally, as a frequency selection filter (below, also referred to as a “filter”) used in an RF (radio frequency) stage of a mobile phone, car phone, or other mobile communication device, a SAW filter has been widely used. In general, as the characteristics required for a frequency selection filter, there can be mentioned a broad pass band, low loss, high amount of attenuation, and other characteristics.
FIG. 8 shows a plan view of an electrode structure of a conventional resonator type SAW filter. In the conventional SAW filter shown in FIG. 8, a first SAW element 91 and a second SAW element 92 are cascade-connected in two stages, an unbalanced input terminal 94 is connected to an IDT electrode 82 located at the center of the first SAW element 91, and an unbalanced output terminal 95 is connected to an IDT electrode 85 located at the center of the second SAW element 92 (see, for example, Patent Document 1).
In this way, by cascade-connecting resonator electrode patterns in two stages, interference of standing waves of the first stage and the second stage occurs. This enables a high attenuation of an amount of out-of-band attenuation, and the amount of out-of-band attenuation of the filter characteristic can be improved. Namely, by employing a configuration in which SAW filters having the same characteristics are cascade-connected in two stages, the signal attenuated in the first stage is further attenuated in the second stage, so the amount of out-of-band attenuation can be improved about two-fold.    Patent Document 1: Japanese Patent Publication (A) No. 2002-84163